We’ve had a discussion about this very same issue with another customer earlier this week.
The specs have changed, and that’s why we also changed the way we process the feature. Right now it says these features should only work with characters as defined in ArabicShaping.txt. Since we don’t know how that should work (with non Latin scripts, first and last have a different meaning), it is now not implemented at all.
Also see this topic at our forum:
How to make a contextual cursive font